Offsetting Bearing Walls

Q.There's a framing rule of
thumb I've used for years that says it's okay to offset
a second-story bearing wall from the first-story
bearing wall below, as long as you don't offset it by
more than the depth of the joist. We use this rule for
standard dimensional lumber, but would it also apply to
wood I-joists?

A.Gary Schweizer, PE, a
senior engineer with iLevel by Weyerhaeuser, in
Charlotte, N.C., responds: Wood I-joists
actually need to be designed to account for the
offset wall load. The rule of thumb you mention is
a code provision (2006 IRC 502.4) for conventional
construction and is applicable to all rectangular
joists (sawn lumber, structural composite lumber,
glulams, and so on), but it doesn't apply to
I-joists.

Generally speaking, sawn-lumber joist spans are
limited by bending or deflection in the middle of
the span, rather than by shear or bearing at a
support; the additional load due to the offset wall
does not result in an overstress condition.

However, an I-joist by design is optimized for
material usage and performance, which results in
conditions where the spans may be limited by shear
or bearing capacity.

Because the added load from an offset wall could
result in an overstress condition, most I-joist
manufacturers recommend stacking bearing walls
directly over supports below and including blocking
panels or squash blocks (vertical 2x4s) between the
joists to transfer the load from the wall
above.